The present invention relates generally to the bonding of materials, and more particularly to adhesive bonding of deformable materials.
Bonded materials, i.e., a plurality of substrates adhered by adhesive, are found in goods of various types. Plywood is but one example of bonded material well known to the public. Bonded materials, such as plywood, that are intended to have great strength, may require high strength adhesives. High performance thermoset or thermoplastics adhesives, which are devoid of solvent, are used in most high performance bonding applications. Thus, processing is often carried out at a relatively high temperature sufficient to activate the adhesive. High strength materials are typically processed under conditions, such as high temperature and pressure, to ensure that a strong bond is formed. Indeed, bonding, whether for consumer and durable goods or defense and aerospace applications, is one of the most demanding processes in industry.
Shoes, particularly athletic shoes, often comprise bonded materials. In particular, a sole assembly, i.e., the midsole and outsole, often are bonded together. Bonding of the midsole with the outsole is particularly important.
Whereas high performance thermoset or thermoplastics adhesives are used in most high performance bonding applications, solvent-based adhesives are used in most applications involving bonding of low grade materials, typically for both consumer and durable goods. Bonding of outsoles and midsoles in athletic shoe manufacturing is an example of such an application. However, solvent based adhesives may be environmentally objectionable. A bonding process using solvent based adhesive typically is a slow process, not only because evaporation of solvent takes time, but also because adhesives often must be applied several times on both substrates to form the required bond.
A solvent-based process thus may require a long processing time and care may need to be taken to prevent environmental damage and deleterious health effects caused by solvents. Therefore, it is preferable to use a high performance adhesive, such as a thermoplastic (e.g., hot melt) or a thermoset adhesive, for these bonding applications. A bonding process using a solventless adhesive is a xe2x80x9cgreenxe2x80x9d process, i.e., is environmentally unobjectionable. Such a process also typically is relatively quicker because it can be automated. Typically, these high performance adhesives require only one application and can be rapidly activated with appropriate heat sources.
Unfortunately, most materials used in consumer and durable goods require high performance at relatively low temperatures (typically xe2x88x9230 to 80xc2x0 C.), and tend to deform at the activation temperatures of such adhesives if the entirety of the substrate is heated to the requisite temperature.
Materials that deform at a temperature below the activation temperature of an adhesive are difficult to heat as an entirety to a temperature sufficient to form the required bond. Therefore, a selective heating technique, such as single-mode or multi-mode fixed frequency microwave irradiation, has been suggested. However, microwave processing of materials to be bonded has not yet met with much commercial success due to the non-uniform energy distribution inside microwave cavities powered with fixed frequencies. The electromagnetic energy distribution is directly related to the magnitude and configuration of the electric field pattern inside the microwave cavity and is usually localized to specific regions. It is this non-uniform distribution of electromagnetic energy that results in problems during microwave processing, such as non-uniform processing, and difficulties in implementing such processing, such as size limitations on the materials to be bonded.
A fixed frequency microwave signal launched within a microwave cavity is reflected a plurality of times, eventually establishing modal patterns of energy distribution. The overall distribution of electromagnetic energy is not uniform throughout the microwave cavity, resulting in both high and low energy field areas, i.e., hot and cold spots. Microwave heating using multiple modes has succeeded in various food and rubber related applications where the tolerance for thermal gradient is high. Whereas thermal gradients produced by nonuniform electromagnetic energy distribution to a certain extent can be accommodated in certain applications, such gradients cannot be tolerated in other materials.
Therefore, there exists a need for a process for bonding materials that deform at a temperature lower than the activation temperature of a high performance adhesive, such as a thermoplastic or a thermoset adhesive.
This invention is directed to a method for bonding materials having low deformation temperatures using thermoplastic or thermoset adhesives, preferably solventless adhesives. In particular, the invention is directed to bonding materials having deformation temperatures lower than the activation temperature of the adhesive.
In accordance with the method, the materials to be bonded are exposed to energy (e.g., variable frequency microwave irradiation) at frequencies selected to heat the adhesive in preference to heating the substrate material having a low deformation temperature.
According to another aspect of the present invention, an apparatus is provided for applying pressure to a workpiece irradiated with microwave energy. The apparatus includes a rigid frame having opposing top and bottom portions. The frame may be formed from substantially microwave transparent material or may be coated with electrical insulation to suppress arcing when exposed to microwave irradiation.
A diaphragm assembly is disposed between the top and bottom frame portions. The diaphragm assembly includes substantially microwave transparent upper and lower membranes formed from flexible material such as silicone rubber, and the like. The upper and lower membranes are sealed along peripheral edge portions thereof to a manifold to form a chamber between opposing inner surfaces of the upper and lower membranes. The chamber is configured to receive a workpiece and to apply pressure to the workpiece when a vacuum is created within the chamber. At least one of the upper and lower membrane inner surfaces may be provided with a raised pattern for facilitating the removal of air from within the chamber.
The manifold may include at least one aperture for facilitating the removal of air from the chamber. The manifold may also include at least one aperture connected to an air pump or other suitable device for facilitating the movement of air to and from the manifold. Opposing top and bottom portions may be pivotally attached along adjacent edge portions to facilitate placing a workpiece to be processed within the chamber.
Another aspect of the invention relates to a system for bonding a workpiece comprising a plurality of substrates. The system includes a device for applying pressure to a workpiece formed of a plurality of substrates having adhesive disposed therebetween; and a device for irradiating the workpiece with microwave energy to heat the adhesive. The system preferably includes a carrier for supporting the plurality of substrates during the bonding process, preferably in the form of the apparatus for applying pressure to a workpiece as described above; a transport mechanism for moving the carrier to a plurality of stations in the system; an assembly station at which the plurality of substrates having an adhesive between them are assembled into the workpiece; a loading station at which the assembled workpiece is loaded onto said carrier; a pressure application station at which pressure is applied by way of the carrier to hold the workpiece together; and a microwave energy application station at which microwave energy is applied to the workpiece supported in the carrier to bond the substrates together with the adhesive.
The system can further include a cooling station at which the workpiece cools after the application of microwave energy at the microwave energy application station. The transport mechanism preferably includes a conveyer for moving the carrier from the loading station to the pressure application station, and a transfer device for moving the carrier from the conveyer into the microwave energy application station and from the microwave application station to the cooling station. The microwave energy source at the microwave energy application station may irradiate a workpiece with variable frequency microwave energy by sweeping the workpiece with at least one window of microwave frequencies. Each window of microwave frequencies is selected to heat the adhesive essentially without heating the substrates above deformation temperatures thereof.
In a preferred embodiment of the invention, the method and system are used to bond together footwear components such as outsoles, midsoles, and uppers.